Structural phase boundary helps explain property differences between piezoelectric ceramics

The morphotropic phase boundary (MPB) between ferroelectric rhombohedral and tetragonal phases is considered the origin of large piezoelectricity in high-performance materials such as lead zirconate titanate (PZT) and bismuth sodium titanate (BNT).

MPBs of BNT-based piezoelectric ceramics possess an unexpectedly large high-field-induced electrostrain compared to their lead counterparts, as well as feature small piezoelectricity near the phase boundary. In an effort to investigate why such large differences exist between types of ferroelectrics, Yan et al. examined the structural phase boundaries in BNT and PZT ceramics.

Using a combination of dielectric measurements, dynamic mechanical analysis, in-situ x-ray diffraction and transmission electron microscopy, the group discovered a local structural phase boundary associated with oxygen octahedral tilt-untilt transition near the morphotropic phase boundary in the material.

“This work provides a deep insight to understand the origin of different effects of the MPB in various ferroelectrics,” said author Kang Yan. “Thus, it may help us to develop new high-performance piezoelectric materials based on this new understanding of the MPB theory, especially for lead-free piezoelectric materials.”

The local structural change was found to be decoupled with the average structural change of the rhombohedral-to-tetragonal phase transition at MPB. The authors identified this decoupled structural change to be responsible for the large high-field-induced electrostrain and small low-field-induced piezoelectricity in the BNT-based piezoelectric ceramics.

Yan said the group hopes to leverage their findings to find or design an MPB with coupled local structural change of oxygen octahedral tilt-untilt transition in lead-free piezoelectric materials in the future.

Source: “A structural phase boundary due to oxygen octahedral tilt-untilt transition in Bi_0.5Na_0.5TiO₃-based piezoelectric ceramics,” by Kang Yan, Shuai Ren, Fangfang Wang, Dawei Wu, Kongjun Zhu, and Xiaobing Ren, Journal of Applied Physics (2020). The article can be accessed at https://doi.org/10.1063/1.5144639 .